benchmarks/gem_syslatency: Prevent CPU sleeps (C-states)

In order to keep the latency as low as possible for the idle load, we
need to keep the CPU awake. Otherwise we end up with the busy workload
having lower latency than the idle workload!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

benchmarks/.gitignore

@@ -8,6 +8,7 @@ gem_latency
 gem_mmap
 gem_prw
 gem_set_domain
+gem_syslatency
 gem_userptr_benchmark
 intel_upload_blit_large
 intel_upload_blit_large_gtt

benchmarks/gem_syslatency.c

@@ -75,6 +75,16 @@ static void bind_cpu(pthread_t thread, int cpu)
 	pthread_setaffinity_np(thread, sizeof(mask), &mask);
 }
 
+static void force_low_latency(void)
+{
+	int32_t target = 0;
+	int fd = open("/dev/cpu_dma_latency", O_RDWR);
+	if (fd < 0 || write(fd, &target, sizeof(target)) < 0)
+		fprintf(stderr,
+			"Unable to prevent CPU sleeps and force low latency using /dev/cpu_dma_latency: %s\n",
+			strerror(errno));
+}
+
 #define LOCAL_I915_EXEC_NO_RELOC (1<<11)
 #define LOCAL_I915_EXEC_HANDLE_LUT (1<<12)
 
@@ -246,9 +256,11 @@ int main(int argc, char **argv)
 		default:
 			break;
 		}
-
 	}
 
+	/* Prevent CPU sleeps so that busy and idle loads are consistent. */
+	force_low_latency();
+
 	busy = calloc(ncpus, sizeof(*busy));
 	if (enable_gem_sysbusy) {
 		for (n = 0; n < ncpus; n++) {